Boron occurs naturally in the environment because it is released from the Earth's crust into bodies of water through erosion and infiltration processes. The presence of boron in natural water of the northern area of Chile is due to the existing boron deposits, since Chile is the third boron-producing country worldwide and the most important source is located in the evaporite basins known as salt flats, located in the northern area, specifically in Regions XV, I, II and III (Chong et al., 2000).
Water characterization results obtained by various public entities have found boron concentrations in water between 0.28 and 78 [mg/l]. The demand for water in the northern area has increased in recent years (considering only the city of Arica) to the point at which intake restrictions have been created. Studies for creating new supply sources are aimed at the Lluta river basin, but the physicochemical characteristics of the river's effluent render it unviable. Agricultural demands will not be met with the surface water resources, if the rate of growth of arable land is taken into account and it is considered that underground deposits are at their limit for population and industrial demands.
With respect to boron content in water, Chile does not include this element in the drinking water standard. However, the World Health Organization (WHO) recommends a maximum value of 0.5 [mg/l] for water for human intake. Furthermore, humans can also be exposed to boron through fruit and vegetable intake. The excessive intake of boron can cause health problems: vomiting, diarrhea, stomach, liver, kidneys and brain infections; irritation in the nose, throat and eyes. The prolonged intake of boron affects male reproductive organs. Five g of boric acid are required to make a person sick and 20 g can cause death.
The Chilean quality of water for irrigation standard (NCh 1333) establishes a limit of 0.75 [mg/l] of boron. This is because the boron content could generate toxic effects in some crops, affecting quality and yield. Boron toxicity can affect almost all crops depending on individual tolerances, the following crops being the most sensitive to boron: apple, broccoli, brussel sprouts, cabbage, carrot, cauliflower, citrus fruits, celery, clove, conifers, corn, lettuce, onion, apricot, peanut, pear, sunflower, sweet corn, sweet potato, beet, potato, tomato and turnip.
According to various alternatives that have been evaluated, in order to find a solution to the boron problems in the northern area of Chile, the use of crops resistant to the conditions of each area and also an improvement in the quality of irrigation water have been proposed. However, these measures generally mean a limitation on agricultural development since in the first case it limits crop diversification and the most resistant crops usually have a lower cost compared with others that are more sensitive.
In addition, with respect to the improvement of the quality of water some alternatives that have been evaluated are to prevent tributaries from contaminating watercourses, the deviation of natural basins, to prevent the mixture of irrigation water with drainage water, inter alia. However, measures of this type require building expensive civil works and further lead to a drop in water availability, which is critical since there is a water resource shortage in the affected areas.
Another alternative is to remove the contaminant from irrigation water by means of treatment technologies. This solution is possible but requires cost-efficient technologies that can be readily implemented.
Given this scenario, the Water and Environment Directorate of Chile Foundation (Gerencia de Agua y Medio Ambiente de Fundación Chile) started to search for a highly cost-effective technological solution that allowed improving the quality of water in northern Chile.
The technological comparison with respect to removal efficiencies included the analysis of technologies such as: reverse osmosis (40 to 60% of boron removal), electrodialysis (40 to 75%), co-precipitation (60%), adsorption (10 to 30%) and ion exchange and chelating processes (99%). According to the results obtained from the comparative analysis of existing worldwide technologies it was concluded that ion exchange technology based on the use of specific resins was the most efficient technology as regards its capacity to remove boron from water, reaching removal rates of about 99%. However, its operating cost is high due to the cost of the inputs required, particularly the specific ion exchange resin. These results marked the start of the study to develop the treatment system of the present invention. The study included developing columns, identifying lower-cost resins on the international market that afford good boron adsorption results while maintaining their efficiency in removing boron.
In the prior art, patent document JP2005058855 describes a method of recovering high-purity boron that comprises passing the water to be treated containing phosphoric acid and boron through a resin selective for boron adsorption. Then a mineral acid solution is circulated over the resin to remove adsorbed boron and an alkaline solution is circulated to remove acid from the resin.
Patent document JP2003094051 describes a method of regenerating a resin without having to adjust pH by incorporating hydrochloric acid or sulfuric acid. The method of regenerating boron adsorbed in the selective resin comprises eluting the resin with a mineral acid solution and then a NaOH solution is flushed through it. The amount of the NaOH solution to be added is controlled such that the pH of the solution around the resin is from 7.5 to 8.5
Patent document JP2002059154 describes a method of separating boron from a resin containing high adsorbed boron concentration, which comprises using from 0.5 to 2 mol/liter of hydrochloric acid or from 0.3 a 1 mol/liter of sulfuric acid, then neutralizing with an amount of alkali.
Patent document JP2005296953 describes an apparatus for treating boron-containing waste water and the associated method. The apparatus comprises three towers with ion exchange resins and four tanks for accumulating the liquid to be treated. The method comprises passing the boron-containing waste water through the first tower containing a strong acidic cation exchange resin adjusted for a type of H, then all the anions except boron are adsorbed and removed in a second tower having a weak basic anion exchange resin adjusted for a type of OH, then the boron is effectively absorbed and removed in a third ion exchange tower containing a mixture of a strong acidic cation exchange resin adjusted for a type of H and a strong basic anion exchange resin adjusted for a type of OH. The outflow from the third tower is recirculated into the system and is pooled with the boron-containing sewage water flow to be treated.
It can be seen from the prior art can that there are apparatus and processes associated with recovering boron from sewage flows through towers containing ion exchange resins, and there are processes for regenerating resins that comprise extracting the boron adsorbed in the resin.
However, there is still a need for processes and apparatus that allow extracting boron from sewage at lower operating costs and with high extraction efficiency.
The objective of the present invention consists of providing a system and an associated method of extracting boron from sewage that allows obtaining a better cost efficiency balance.
Furthermore, another objective of the present invention is to increase the service life of the ion exchange resin by means of reducing the amount of times it must be regenerated.
Another objective of the present invention is to achieve maximum utilization of the resin through maximum resin saturation, and to thereby be able to extract more highly concentrated boric acid during the step of regeneration.
Likewise, one objective of the present invention is to provide a continuous and modular system that allows removing boron in continuous flows performing boron extraction and resin regeneration in parallel, without having to stop the process during the step of regeneration.